The present invention relates to a photoconductor and the method for producing the same, wherein the photoconductor can be used, for example, for generating electromagnetic terahertz radiation (THz=terahertz).
Imaging methods play an important role in modern medicine. Here, for example, X-radiation is increasingly used, besides ultrasound and nuclear magnetic resonance. However, the ionizing effect of the high-energy X-radiation presents a health hazard. Since the heat radiation of the human body includes weak portions of THz radiation, an alternative for X-radiation can be seen, for example, in the terahertz range between infrared radiation and microwave radiation. As seen in FIG. 1, the THz range lies approximately between 0.1 and 10 THz. As further fields of usage for THz radiation, for example, monitoring or screening systems for air traffic or further concepts in safety engineering, such as a checking fuel tanks for hidden cracks can be considered.
THz systems can be realized, for example, with so-called femtosecond lasers. These lasers only emit light flashes lasting several femtoseconds (fs). If these short light flashes are directed to a so-called photoconductive antenna of a photoconductive semiconductor material and thin conductive traces deposited thereon, THz radiation can be generated by acceleration of electrons excited by the light pulses.
However, femtosecond lasers are very expensive. A further concept for generating terahertz radiation can be obtained via inexpensive lasers or laser diodes, respectively, by so-called photomixing. For generating THz radiation by photomixing, the principle illustrated in FIG. 2 can be used. There, two almost identical laser diodes are used, that only differ slightly in their emitted light frequencies f1 or f2, respectively. The frequency difference Δf between f1 and f2 is adjusted such that the difference frequency Δf is in the THz range. By superimposing the two light fields, a frequency beat having the difference frequency Δf is obtained. The beat signal is transferred to a so-called photoconductive antenna 20. The photoconductive antenna 20 has a photoconductive semiconductor material, such as GaAs (gallium arsenide) or InGaAs (indium gallium arsenide). Due to the superimposed light and the photo effect, movable electrons are at first generated and then accelerated by a voltage applied via electrodes, whereby electromagnetic radiation in the THz range is emitted. The number of generated charge carriers or electron-hole pairs depends on the intensity of the beat signal, i.e. the number of free charge carriers changes periodically with the frequency of the beat signal. The current in the antenna changes with the beat frequency, and thus, an electromagnetic field 22 having the difference frequency Δf lying in the THz range due to appropriate selection of the frequencies of the laser diodes, is emitted.
FIG. 3 shows a schematical cross section of a possible photoconductor 30. The photoconductor 30 comprises an insulating or semi-insulating semiconductor substrate 40, respectively, and a photoconductive thin layer 42 deposited on the semiconductor substrate 40. Electrodes 36a, b are deposited on the massive photoconductive semiconductor layer 42. A schematic illustration of a photoconductive antenna 20 or a THz antenna having a photoconductor, respectively, is shown in top view in FIG. 4.
The THz antenna 20 comprises an insulating substrate 40, on which two conductive traces 32a and 32b are deposited. The same can, for example, be lithographically deposited conductive traces of gold. In the so-called H structure shown exemplarily in FIG. 4, the conductive traces 32a and 32b additionally comprise a central ridge, which has, in its center, a gap of several μm. In the gap of the central ridge, a massive photoconductive semiconductor layer 42 is deposited on the substrate 40, which is connected in an electrically conductive manner to the two traces 32a and 32b via electrodes 36a,b. Via the electrodes 36a,b or the conductive traces 32a,b, a voltage Ub can be applied for transmit operation.
As has already been described above, light with two frequencies f1 and f2 slightly detuned with respect to each other is focused onto the photoconductive semiconductor layer 42 in the gap of the ridge, such that a beat signal in the THz range results. In the photoconductive semiconductor layer 42, free charge carriers are generated by the radiation, which are captured quickly by crystal lattice defects. Thereby, the charge carrier density is proportional to the light intensity. For transmit operation, an electrical field is applied between the electrodes 36a,b, which accelerates the charge carriers towards the electrodes 36a,b. Thereby, a charge carrier current results between the electrodes 36a,b. Since the light intensity is modulated by the beat signal in THz range, also, an alternating current with THz frequencies results. By this alternating current, an electromagnetic field 22 is generated in the THz range, i.e. THz radiation is emitted via the conductive traces 32a,b. For increasing the efficiency of the THz radiation, a highly resistive silicon lens concentrating the emitted THz radiation can be deposited on its exit area, i.e. the ridge gap.
The photoconductive antenna 20 illustrated in FIG. 4 can be used both for transmitting and receiving THz radiation.
For generating and detecting THz radiation by photoconductive antennas or photomixers 20, fast photoconductors having the following characteristics are necessitated:                electrically insulating in the absence of illumination, i.e. high dark resistance,        photosensitivity at certain wavelengths,        high mobility of generated photo charge carriers        fast decay into the insulating state after switching off the illumination.        
Fast decay of the photoconductivity is obtained by introducing point defects representing fast recombination centers into the photoconductive semiconductor layer 42. Prominent examples are so-called LT (low temperature) GaAs layers deposited by molecular-beam epitaxy (MBE) at low temperatures (<200° C.) on GaAs. Molecular-beam epitaxy is a vacuum based depositing method for producing crystalline layers. As consequence of the LT growth, deep impurities result in the form of point defects or impurity clusters, respectively, which cause recombination in the sub-ps range. Hence, LT GaAs is used as standard material for photoconductor based THz antennas (see, for example B. S. Gupta, J. F. Whitaker, G. A. Mourou, “Ultrafast carrier dynamics in III-V semiconductors grown by molecular-beam epitaxy at very low substrate temperatures”, IEEE J. Quantum Electron. 28, 2464, (1992) or D. Mittleman (Editor) “Sensing with Terahertz Radiation”, ISBN 3-540-43110-1 Springer Verlag Berlin-Heidelberg New York, 2003, chapter “Photomixer for Continuous-Wave Terahertz Radiation of S. M. Duffy, S. Verghese, K. A. McIntosh, pp. 190, and I. S. Gregory, C. Baker, W. R. Tribe, I. V. Bradley, M. J. Evans, E. H. Linfield, A. G. Davies, M. Missous, “Optimization of photomixers and antennas for continuous-wave terahertz emission”, IEEE J. Quantum Electronics, Vol. 41, 717 (2005)).
A general disadvantageous consequence of this technology is that the mobility of charge carriers in the photoconductor decreases significantly, since the impurities also act as scattering centers for the free charge carriers. A further negative effect of crystal defects in the semiconductor is that band edges of energy bands become slurred and blurred, such that the absorption behavior at light radiation in this spectral range becomes less favorable.
GaAs is only sensitive and applicable in the spectral range having wavelengths below 850 nm. Thus, a specific problem for the exemplary spectral range having wavelengths of 1 μm to 1.6 μm results from the fact that the starting material InGaAs on InP (indium phosphide) useful as photoconductor partly behaves differently than LT-GaAs during LT growth. On the one hand, the crystal defects for trapping the electrodes and fast recombination result as well, but in parallel, charge carrier concentration and dark conductivity rise so sharply (see H. Küinzel, J. Böttcher, R. Gibis, and G. Urmann, “Material Properties of In0.53 Ga0.47 As on InP by Low-Temperature Molucular Beam Epitaxy” Appl. Phys. Lett. Vol. 61, 1347 (1992)) that a usage, for example in THz antennas, is no longer useful. Thus, for the material system InGaAs on InP, alternative techniques have been developed for generating crystal defects and fast recombination centers, e.g. the implantation of Fe atoms (see M. Suzuki and M. Tonouchi, “Fe-implanted InGaAs photoconductive terahertz detectors triggered by 1.56 μm femtosecond optical pulses”, Appl. Phys. Lett., Vol. 86, 163504 (2005) or the bombardment with Br ions (see N. Chimot, J. Mangeney, L. Joulaud, P. Crozat, H. Bernas, K. Blary, and J. F. Lampin, “Terahertz radiation from heavy-ion-irradiated In0.53Ga0.47As photoconductive antenna excited at 1.55 μm” Appl. Phys. Lett. vol. 87, 193510 (2005)). However, comparatively good results as with LT-GaAs have not been obtained, and hence, currently, there are no acceptable THz antennas based on photoconductors for the spectral range at 1.5 μm wavelength, which, on the other hand, is very attractive, since here inexpensive semiconductor lasers and numerous fiber components are available.
LT growth of InGaAs on GaAs has also been tested (C. Baker, I. S. Gregory, M. J. Evans, W. R. Tribe, E. H. Linfield, M. Missous, “All-optoelectronic terahertz system using low-temperature-grown InGaAs photomixers”, Optics Express Vol. 13, 9639, (2005)), wherein, however, in this material system, the wavelength area can only be shifted slightly beyond 1 μm, and compared to LT-GaAs considerably poorer characteristics have been obtained.